Rare earth (RE) elements (RE = La-Lu, Sc, Y) are critical components of green technology that include permanent magnets, lighting phosphors, and electric car batteries. In recent years, a monopolized market has led to large variations and unpredictability in the prices of these elements which sparked interest in developing methods toward recycling from end of life technology. Our group has shown that the tripodal hydroxylamine proligand tris(2-tert-butylhydroxy-aminato)benzylamine, H3TriNOx, is capable of separating mixtures of REs by a mechanism of self-association in non-polar solvents. Due to the water-sensitive nature of this system, we targeted a new ligand framework to form air- and water-stable complexes based on a hydroxypyridone framework: tris[(1-hydroxy-2-oxo-1,2-dihydropyridine-3-carboxamido)ethyl]amine, H3tren-1,2,3-HOPO. Interestingly, we observed a precipitation dependence for different rare earth complexes based on pH and thus set out to quantitatively assess this potential separation method. We developed a high-throughput screening technique to monitor the precipitation of RE(tren-1,2,3-HOPO) complexes as a function of pH as well as equivalents of ligand added for selected REs (RE = La, Nd, Dy). In general, we found lower amounts of RE(tren-1,2,3-HOPO) precipitated from solution with decreasing pH (increasing acidity), and increased precipitation with increasing equivalents of ligand. Remarkably, La(tren-1,2,3-HOPO) failed to precipitate at acid concentrations as low as 0.25 M HCl (pH ~0.6), whereas Nd(tren-1,2,3-HOPO) precipitated from solution until 1.50 M HCl (pH ~-0.2) was reached. The complex Dy(tren-1,2,3-HOPO) precipitated under all conditions screened indicating that separations using this system are viable. These separations were performed on mixtures of La/Nd and Nd/Dy under the conditions that showed the largest difference in precipitation between the two REs, giving optimized separation factors of SNd/Dy = 213 ± 34 and SLa/Nd = 16.2 ± 0.2. The separations achieved were comparable to CYANEX ® 302, a commercially competent RE separator, for metal mixtures exclusively in aqueous conditions without the use of any organic diluent.

High-throughput screening to achieve benchtop separations for selected rare earth elements

2019 Fall National Meeting

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